Method for transferring micro device

ABSTRACT

A method for transferring a micro device is provided. The method includes: preparing a carrier substrate with the micro device thereon, wherein an adhesive layer is present between and in contact with the carrier substrate and the micro device; picking up the micro-device from the carrier substrate by a transfer head; forming a liquid layer on a receiving substrate; and placing the micro device over the receiving substrate so that the micro device is in contact with the liquid layer and is gripped by a capillary force.

BACKGROUND

Field of Invention

The present disclosure relates to a method for transferring a microdevice from a carrier substrate to a receiving substrate.

Description of Related Art

In the recent years, light-emitting diodes (LEDs) have become popular ingeneral and commercial lighting applications. As light sources, LEDshave many advantages including low energy consumption, long lifetime,small size, and fast switching, and hence conventional lighting, such asincandescent lighting, is gradually replaced by LED lights.

Traditional technologies for transferring of devices include transfer bywafer bonding from a transfer wafer to a receiving wafer. One suchimplementation is “direct printing” involving one bonding step of anarray of devices from a transfer wafer to a receiving wafer, followed byremoval of the transfer wafer. Another such implementation is “transferprinting” which involves two bonding/de-bonding steps. In transferprinting, a transfer wafer may pick up an array of devices from a donorwafer, and then bond the array of devices to a receiving wafer, followedby removal of the transfer wafer.

SUMMARY

According to some embodiments of the present disclosure, a method fortransferring a micro device is provided. The method includes: preparinga carrier substrate with the micro device thereon, wherein an adhesivelayer is present between and in contact with the carrier substrate andthe micro device; picking up the micro-device from the carrier substrateby a transfer head; forming a liquid layer on a receiving substrate; andplacing the micro device over the receiving substrate so that the microdevice is in contact with the liquid layer and is gripped by a capillaryforce.

It is to be understood that both the foregoing general description andthe following detailed description are by examples, and are intended toprovide further explanation of the disclosure as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure can be more fully understood by reading the followingdetailed description of the embodiment, with reference made to theaccompanying drawings as follows:

FIG. 1 is a flow chart of a method for transferring a micro device froma carrier substrate to a receiving substrate in some embodiments of thepresent disclosure;

FIG. 2 is a schematic cross-sectional view of an intermediate step ofthe method for transferring the micro device in some embodiments of thepresent disclosure;

FIG. 3 is a schematic cross-sectional view of an intermediate step ofthe method for transferring the micro device in some embodiments of thepresent disclosure;

FIG. 4A is a schematic cross-sectional view of an intermediate step ofthe method for transferring the micro device in some embodiments of thepresent disclosure;

FIG. 4B is a schematic cross-sectional view of an intermediate step ofthe method for transferring the micro device in some embodiments of thepresent disclosure;

FIG. 5A is a schematic cross-sectional view of an intermediate step ofthe method for transferring the micro device in some embodiments of thepresent disclosure;

FIG. 5B is a schematic cross-sectional view of an intermediate step ofthe method for transferring the micro device in some embodiments of thepresent disclosure;

FIG. 6 is a schematic cross-sectional view of an intermediate step ofthe method for transferring the micro device in some embodiments of thepresent disclosure;

FIG. 7 is a schematic cross-sectional view of an intermediate step ofthe method for transferring the micro device in some embodiments of thepresent disclosure;

FIG. 8A is a schematic cross-sectional view of an intermediate step ofthe method for transferring the micro device in some embodiments of thepresent disclosure; and

FIG. 8B is a schematic cross-sectional view of an intermediate step ofthe method for transferring the micro device in some embodiments of thepresent disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to the present embodiments of thedisclosure, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers are used in thedrawings and the description to refer to the same or like parts.

In various embodiments, description is made with reference to figures.However, certain embodiments may be practiced without one or more ofthese specific details, or in combination with other known methods andconfigurations. In the following description, numerous specific detailsare set forth, such as specific configurations, dimensions andprocesses, etc., in order to provide a thorough understanding of thepresent disclosure. In other instances, well-known semiconductorprocesses and manufacturing techniques have not been described inparticular detail in order to not unnecessarily obscure the presentdisclosure. Reference throughout this specification to “one embodiment,”“an embodiment” or the like means that a particular feature, structure,configuration, or characteristic described in connection with theembodiment is included in at least one embodiment of the disclosure.Thus, the appearances of the phrase “in one embodiment,” “in anembodiment” or the like in various places throughout this specificationare not necessarily referring to the same embodiment of the disclosure.Furthermore, the particular features, structures, configurations, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

The terms “over,” “to,” “between” and “on” as used herein may refer to arelative position of one layer with respect to other layers. One layer“over” or “on” another layer or bonded “to” another layer may bedirectly in contact with the other layer or may have one or moreintervening layers. One layer “between” layers may be directly incontact with the layers or may have one or more intervening layers.

FIG. 1 is a flow chart of a method for transferring a micro device froma carrier substrate to a receiving substrate. FIGS. 2 to 7 are schematiccross-sectional views of intermediate steps of the method 100 of FIG. 1.References are made to FIGS. 1 to 7. The method 100 begins withoperation 110 in which a carrier substrate 210 is prepared with a microdevice 220 thereon. An adhesive layer 230 is present between and incontact with the carrier substrate 210 and the micro device 220(referring to FIG. 2). The method 100 continues with operation 120 inwhich the micro device 220 is picked up from the carrier substrate 210by a transfer head 240 (referring to FIG. 3). The method 100 continueswith operations 130 and 140 in which a liquid layer 250 or a patternedliquid layer 250 is formed on a receiving substrate 260 (referring toFIGS. 4A and 4B), and then the micro device 220 which has been picked upis placed over the receiving substrate 260, so that the micro device 220is in contact with the liquid layer 250 and is gripped by a capillaryforce produced by the liquid layer 250 (referring to FIGS. 5A, 5B, and6). After the gripping, the transfer head 240 is detached from the microdevice 220, and the micro device 220 remained on the receiving substrate260 and is substantially held in a position within a controllable regionon the receiving substrate 260 (referring to FIG. 7).

Although in previous paragraph and FIG. 1 only “a” micro device 220 isillustrated, “multiple” micro devices 220 may be used in practicalapplications and is still within the scope of the present disclosure, aswill be illustrated in the following embodiments.

Reference is made to FIG. 2. As mentioned above, the adhesive layer 230is present between the carrier substrate 210 and a plurality of microdevices 220. Specifically, the adhesive layer 230 is in contact with thecarrier substrate 210 and the micro devices 220. In some embodiments,the formation of the adhesive layer 230 is performed by coating anadhesive capable material onto the carrier substrate 210. The adhesivelayer 230 may be coated by a spin coater, a slit coater, or anycombination thereof. In some embodiments, the adhesive layer 230 may bemade of an adhesion capable organic material, such as epoxy,polymethylmethacrylate (PMMA), polysiloxanes, silicone, or anycombination thereof. Furthermore, the adhesive layer 230 may have athickness in a range from about 1 μm to about 100 μm.

An adhesive force f_(AD) is an adhesive force of the adhesive layer 230to each of the micro devices 220 after reduction. Said reduction isreducing an original adhesive force of the adhesive layer 230 to each ofthe micro devices 220, which may be performed before some of the microdevices 220 are picked up. In some embodiments, the reduction may beperformed by applying an electric field, an electromagnetic radiation,heat, ultrasound, a mechanical force, a pressure, or any combinationthereof on the adhesive layer 230, and should not be limited thereto. Insome embodiments, a lateral length of one of the micro devices 220 isabout 1 μm to 100 μm. For example, for one micro device 220 with asurface area about 10 μm×10 μm, said reduced adhesive force f_(AD) isabout 50 nanonewton (nN). Embodiments of this disclosure are not limitedthereto. Proper modifications to the adhesive layer 230 depending on anactual application may be performed. The adhesive force f_(AD) mayinclude van der Waals forces, but should not be limited thereto.

In some embodiments, the carrier substrate 210 may be a rigid substrate.The rigid substrate may be made of glass, silicon, polycarbonate (PC),acrylonitrile butadiene styrene (ABS), or any combination thereof.Embodiments of this disclosure are not limited thereto. Propermodifications to the carrier substrate 210 depending on an actualapplication may be performed.

In some embodiments, the micro devices 220 may be a light emittingstructure such as a compound semiconductor having a bandgapcorresponding to a specific region in the spectrum. For example, thelight emitting structure may include one or more layers based on II-VImaterials (e.g. ZnSe, ZnO) or III-V nitride materials (e.g. GaN, AlN,InN, InGaN, GaP, AlInGaP, AlGaAs or alloys thereof). In some otherembodiments, the micro devices 220 may also be integrated circuits (IC)or microelectromechanical system (MEMS) devices, and should not belimited thereto.

Reference is made to FIG. 3. As mentioned above, some of the microdevices 220 are picked up from the carrier substrate 210 by the transferhead 240. In some embodiments, the transfer head 240 may exert apicking-up pressure or a picking-up force on each of the micro devices220 by way of, for example, vacuum, adhesion, magnetic attraction,electrostatic attraction, or the like. Hereinafter only adhesive forcewill be discussed, but other types of forces mentioned above are stillwithin the scope of the present disclosure. In some embodiments, thetransfer head 240 may have a plurality of grip regions 242 for pickingup the micro devices 220. There may also be recesses 244 present betweenthe grip regions 242. The grip regions 242 of the transfer head 240 maybe made of an adhesion capable material or, the transfer head 240 mayhave a patterned adhesive layer thereon, such that when the transferhead 240 is in contact with the micro devices 220, each of the microdevices 220 may be picked up by an adhesive force f_(TD). In someembodiments, for one micro device 220 with a surface area about 10 μm×10μm, said adhesive force f_(TD) is about 100 nN to 1000 nN for one microdevice 220. The adhesive force f_(TD) may include van der Waals forces,but should not be limited thereto.

As mentioned above, in some embodiments, the original adhesive force maybe reduced before the picking up to form the adhesive force f_(AD), sothat the adhesive force f_(TD) is greater than the adhesive force f_(AD)when the picking up is performed.

Reference is made to FIGS. 4A and 4B. As mentioned above, the liquidlayer 250 is formed on the receiving substrate 260. The liquid layer 250may be formed to be one layer on the receiving substrate 260 as shown inFIG. 4A or patterned to be discrete portions on the receiving substrate260 as shown in FIG. 4B. In FIG. 4B the patterned liquid layer 250 maybe the position where the micro devices 220 will be placed over. Thereceiving substrate 260 may be a display substrate, a lightingsubstrate, a substrate with functional devices such as transistors orintegrated circuits, or a substrate with metal redistribution lines, butshould not be limited thereto. In some embodiments, the liquid layer 250may be formed by lowering a temperature of the receiving substrate 260in an environment including a vapor, such that at least a part of thevapor is condensed to form the liquid layer 250 on the receivingsubstrate 260. In some embodiments, the temperature of the receivingsubstrate 260 is lowered to about the water dew point, such that thewater vapor in the environment is condensed to form liquid water servingas the liquid layer 250. In these embodiments, the liquid layer 250includes water. In alternative embodiments, the liquid layer 250 mayinclude methyl alcohol, ethanol, glycerol, combinations thereof, or thelike. Furthermore, the formation of the liquid layer 250 may also beachieved by inkjet printing, roller coating, dip coating, or the like.

References are made to FIGS. 5A, 5B and 6. As mentioned above, the microdevices 220 which have been picked up are placed over the receivingsubstrate 260 by the transfer head 240, so that each of the microdevices 220 is in contact with the liquid layer 250 and gripped by acapillary force f_(CF). Specifically, the micro devices 220 are placedin close proximity to the receiving substrate 260, so that the liquidlayer 250 can grip the micro devices 220. The meniscuses 252 of theliquid layer 250 as shown in FIG. 6 are caused by the capillary forcef_(CF). The micro devices 220 are gripped by the capillary force f_(CF)produced by the liquid layer 250 between the micro devices 220 and thereceiving substrate 260. In some embodiments, a thickness of the liquidlayer 250 is less than a thickness of the micro devices 220 when themicro devices 220 are gripped by the capillary force f_(CF). In someembodiments, the receiving substrate 260 may further include at leastone conductive pad thereon, and one of the micro devices 220 is in closeproximity to the conductive pad and is gripped by the capillary forceproduced by the liquid layer 250 between one of the micro devices 220and the conductive pad.

Reference is made to FIG. 7. As mentioned above, the micro devices 220are detached from the transfer head 240 after the micro devices 220 aregripped by the capillary force f_(CF). In some embodiments, the microdevices 220 are adhered to the adhesive layer 230 by an adhesive forcef_(AD). Some of the micro devices 220 are attached to the transfer head240 by the adhesive force f_(TD), and the capillary force f_(CF) isgreater than the adhesive force f_(TD) so that the micro devices 220 aredetached from the transfer head 240 and are stuck to the receivingsubstrate 260 when the placing is performed.

Specifically, a lateral length of one of the micro devices 220 may beless than 40 μm, but should not be limited thereto. One of the microdevices 220 may also be a cylindrical, a triangular, a cuboid, ahexagonal, an octagonal, or a polygon in other embodiments. As a result,the capillary force f_(CF) is greater than the adhesive force N_(TD) dueto a size effect of the micro devices 220. Specifically, regarding themicro devices 220 with a size within about a range mentioned above, thecapillary force f_(CF) becomes a dominant force since an influence of acapillary force on an object will gradually dominate other forcesapplied on the object when a size of the object gradually scales down.More specifically, the forces applied to the micro devices 220 with thesize within the range mentioned in these embodiments will obey thefollowing inequality:f_(AD)<f_(TD)<f_(CF)  (1)wherein the left-hand side of inequality (1) f_(AD)<f_(TD) may besatisfied by choosing suitable combination of materials for the adhesivelayer 230 and surfaces of the grip regions 242 in contact with the microdevices 220 respectively. Generally, the adhesive forces f_(AD) andf_(TD) per unit area do not change depend on a size of the micro devices220. In some embodiments, the adhesive force f_(TD) may be additionallymodified by the speed of moving up the transfer head 240 away from thecarrier substrate 210 after the transfer head 240 is in contact with themicro devices 220. The faster the speed is, the greater the adhesiveforce f_(TD) is. As such, transfer processes mentioned above may beachieved with adhesive type transfer head 240. Complicated circuitdesigns or mechanical designs for transfer heads operated byelectrostatics force, vacuum force, mechanical force, or any combinationthereof can be omitted. An adhesive type transfer head 240 is capable ofcompleting the transfer processes, and the cost of the processes isreduced.

After completing the transfer processes, the liquid layer 250 isevaporated, such that at least one micro device 220 is bound to thereceiving substrate 260 or at least one conductive pad on the receivingsubstrate 260. The micro devices 220 may have electrodes thereonrespectively for electrically contacting with the receiving substrate260 via the conductive pads. Details of conductive pads and electrodesare omitted herein. The evaporation of the liquid layer 250 may beachieved by, for example, raising a temperature of the receivingsubstrate 260 or the conductive pads. After the liquid layer 250 isevaporated, the micro devices 220 are stuck to the receiving substrate260.

In the above embodiments supported by FIGS. 1 to 7, after some of themicro devices 220 are placed over the receiving substrate 260, the microdevices 220 are gripped by the capillary force f_(CF) produced by theliquid layer 250 between the micro devices 220 and the receivingsubstrate 260. As such, an adhesive type transfer head 240 withoutcomplicated circuit design is capable of completing the transferprocesses due to the presence of the liquid layer 250, and the cost ofthe processes are reduced.

References are made to FIGS. 8A and 8B. FIGS. 8A and 8B are schematiccross-sectional views of intermediate steps of the method 100illustrated in FIG. 1. In alternative embodiments, at least one of themicro devices 220 has a photoresist layer 222 thereon. The photoresistlayer 222 may be optionally needed during a fabrication process such asdicing. The photoresist layer 222 is coated onto at least one of themicro devices 220 to act as a mask for dicing after said photoresistlayer 222 is patterned. The photoresist layer 222 may be a positivephotoresist layer or a negative photoresist layer, but should not belimited thereto. In the embodiments, the photoresist layer 222 ispresent on the micro devices 220 before the picking up (referring toFIG. 8A), and the micro devices 220 are attached to the transfer head240 via the photoresist layer 222 when the picking up is performed(referring to FIG. 8B).

As a result, the method 100 mentioned above supported by FIG. 1 alsoapplies to the present embodiments since the capillary force f_(CF) isstill greater than an adhesive force f_(TA) between the photoresistlayer 222 and the transfer head 240. Specifically, the micro devices 220are adhered to the adhesive layer 230 by an adhesive force f_(AD), themicro devices 220 are attached to the transfer head 240 via thephotoresist layer 222 by the adhesive force f_(TA), and the capillaryforce f_(CF) is greater than the adhesive force f_(TA), so that themicro devices 220 are detached from the transfer head 240 and are stuckto the receiving substrate 260 when the placing is performed. Theadhesive forces f_(TA) may include van der Waals forces, but should notbe limited thereto. During the process of transferring, forces mentionedabove obey the following equation:f_(AD)<f_(TA)<f_(CF)  (2)wherein the left-hand of inequality (2) f_(AD)<f_(TA) may be satisfiedby choosing suitable combination of materials for the adhesive layer230, the photoresist layer 222, and a surface of the grip region 242 ofthe transfer head 240 respectively. For example, the adhesive layer 230may be made of aforementioned materials. The grip region 242 may be madeof aforementioned materials for the adhesive layer 230 andpolydimethylsiloxane. The photoresist layer 222 may be made of acrylicresin or novolak resin.

Although the present disclosure has been described in considerabledetail with reference to certain embodiments thereof, other embodimentsare possible. Therefore, the spirit and scope of the appended claimsshould not be limited to the description of the embodiments containedherein.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentdisclosure without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present disclosurecover modifications and variations of this disclosure provided they fallwithin the scope of the following claims.

What is claimed is:
 1. A method for transferring a micro device,comprising: preparing a carrier substrate with the micro device thereon,wherein an adhesive layer is present between and in contact with thecarrier substrate and the micro device, wherein the micro device isadhered to the adhesive layer by a first adhesive force; picking up themicro device from the carrier substrate by a transfer head, wherein themicro device is attached to the transfer head by a second adhesiveforce; forming a liquid layer on a receiving substrate; and placing themicro device over the receiving substrate so that the micro device is incontact with the liquid layer and is gripped by a capillary force,wherein a lateral length of the micro device is less than 40 μm, therebyresulting in that the capillary force is greater than the first adhesiveforce and the second adhesive force so that the micro device is detachedfrom the transfer head and is stuck to the receiving substrate when theplacing is performed.
 2. The method of claim 1, wherein the firstadhesive force and the second adhesive force comprise van der Waalsforces.
 3. The method of claim 1, wherein the second adhesive force isgreater than the first adhesive force.
 4. The method of claim 1, whereina photoresist layer is present on the micro device before the pickingup, and the micro device is attached to the transfer head via thephotoresist layer when the picking up is performed.
 5. The method ofclaim 4, wherein the micro device is adhered to the adhesive layer by afirst adhesive force, the micro device is attached to the transfer headvia the photoresist layer by a third adhesive force, and the capillaryforce is greater than the first adhesive force and the third adhesiveforce so that the micro device is detached from the transfer head and isstuck to the receiving substrate when the placing is performed.